Valve metals such as tantalum and niobium are characterized by a stable oxide coating which imparts useful properties such as corrosion resistance and electrical resistance which make such metals attractive for various applications such as an anode material in electrolytic capacitors and as a material of construction in certain corrosive environments. The oxide coating can be a hindrance in bonding valve metals, e.g. in sintering pressed valve metal powders into a pellet bonded to a valve metal lead wire or lead tab. As finer valve metal powders are used to achieve higher capacitance, lower sintering temperatures are used to prevent excessive loss of surface area from exposure to sintering temperatures. Moreover, as satisfactory particle to particle bonding is achieved in the pressed powder, the particle to lead connection bond can be deficient at lower sintering temperatures allowing the sintered pellet to be readily pulled off of the lead wire or lead tab during fabrication or use of the capacitor.
This invention provides valve metal including a second metal where the second metal is preferably at a greater concentration at or near the peripheral portion of the valve metal than within the valve metal. Such valve metal may be in a variety of forms, e.g. wire, sheet including foil, or particulate including flaked, nodular or angular powder. In such forms, the second metal is concentrated in the valve metal at the periphery of the valve metal form or valve metal article. Valve metal wire according to this invention is useful in the fabrication of anodes for electrolytic capacitors to provide enhanced bonding strength between such wire and sintered pellets of valve metal powder. Valve metal sheet according to this invention is similarly useful to form lead tabs on sintered valve metal powder anodes.
This invention also provides a method of adding a second metal to valve metal comprising:
(a) coating a valve metal surface with a solution comprising a second metal; and
(b) heating said valve metal and coating in the presence of an oxygen getter at a temperature sufficient to remove oxygen from said valve metal and to provide said valve metal with said second metal in a peripheral region of said valve metal. Preferably, the second metal is provided at or near to the surface of the valve metal, e.g. in a peripheral region, to improve a surface property of the valve metal.
As used herein the term xe2x80x9cvalve metalxe2x80x9d refers to tantalum, niobium, alloys of valve metals including alloys of tantalum and niobium, and valve metals including grain growth inhibitors such as tantalum with up to about 1000 ppm of silicon or yttrium oxide, niobium with zirconium and the like. Unlike the second metal which is preferably concentrated at a peripheral region, other additives to a valve metal such as grain growth inhibitors and alloying elements are more uniformly distributed throughout the valve metal.
As used herein the term xe2x80x9csecond metalxe2x80x9d refers to a metal component in a valve metal which is not uniformly distributed throughout the valve metal. Useful second metals can be found in Groups IVB, VB, VIB, VIIB, and VIII of the periodic table and include tantalum, niobium, nickel, titanium, zirconium, tungsten and iron. Nickel is a preferred second metal for tantalum articles to be bonded to sintered tantalum powders.
In the method of this invention a second metal is added to a valve metal by coating the valve metal surface with a compound of the second metal and heating the coated valve metal in the presence of an oxygen getter at a temperature sufficient to remove oxygen from said valve metal and provide the metal in the peripheral region of the valve metal. The getter should also be effective in removing anions associated with the second metal in the coating. This method is especially useful in preparing valve metal wire, powder and sheet with a second metal.
When such second metal-containing wire is used as lead wire for preparing sintered anodes of pelletized valve metal powder, higher bond strength between the sintered powder and lead wire can be achieved. Such wire is expected to be useful with valve metal powder which is preferably sintered at low temperatures, e.g. less than 1500xc2x0 C. for tantalum powders or lower such as in the range of 1200 to 1400xc2x0 C.
It is expected that the improved bonding of an anode pellet to an anode lead wire in accordance with the present invention will reduce (and thereby improve) the susceptibility of the anode to inrush surge current. While not wishing to be bound by theory, it is understood that inrush surge current, as occurs when a discharged circuit is initially energized, can cause disruption of tantalum capacitors if the contacts between powder and wire are poorly established and therefore of small total cross-section area. The high instantaneous current forced to traverse poorly developed bonds results in high current density and ohmic heating in the area of the wire-anode interface. The high current density and heat generation can result in capacitor failure and possibly even ignition of the valve metal pellet. The improved wire to pellet bonding provided by this invention is expected to reduce the occurrence of disruptions caused by current surge.
Valve metal sheet according to this invention is also expected to be useful in making lead tabs for sintered anodes of high capacitance valve metal powder. Thus, preferred articles of this invention are valve metal wires and sheets having a second metal which improves a surface property, e.g. bonding to sintered bodies of valve metal powder. Such lead wires commonly have diameters in the range of about 100 to 1000 micrometers (xcexcm); sheets may be thinner, e.g. on the order of 50 xcexcm; and foil, even thinner. Second metal-containing powders according to this invention may be useful as bonding agents for joining valve metal parts or for low temperature sintering valve metal powders or simply providing metal additives, e.g. dopants, to valve metals in peripheral regions.
Preferred wire and sheet provided by this invention have a second metal concentrated in the peripheral region of the valve metal article where the thickness of the peripheral region will depend on the diffusion rate of the second metal into the valve metal. Factors influencing diffusion are expected to include the concentration per unit surface area of the second metal in the coating and the time and temperature at which the valve metal is subjected to thermal treatment as with exposure to the oxygen getter. In many cases it is preferable that the peripheral region in which the second metal is concentrated be as thin as possible to provide enhanced surface properties without affecting bulk properties of the valve metal. Another benefit of a thin peripheral region of concentrated second metal is that the second metal concentration can be reduced to minimal levels after the enhanced surface properties have been utilized, e.g. by thermal treatment to facilitate diffusion of the second metal into the bulk of the valve metal which has a low concentration, if any, of the second metal, e.g. into the core region of a wire or center region of a sheet. In preferred aspects of this invention the peripheral region of concentrated second metal can be within 1 to 2 micrometers from the surface. For some applications the second metal concentration in the peripheral region can be up to about 1 weight percent or more. Based on the total weight of valve metal in an article, the bulk concentration of the second metal can be in the range of 2 to 2000 ppm. For some applications bulk concentration of the second metal can be 5 to 200 ppm. For other application the bulk concentration of the second metal can be 5 to 50 ppm.
A valve metal wire in accordance with the invention can be annealed (55,000-90,000 psi), unannealedxe2x80x94half hard (105,000-160,000), or unannealedxe2x80x94hard (130,000-215,000 psi).
A useful second metal for tantalum is nickel which is stable and resistant to corrosion. The amount of nickel used in tantalum wire will preferably be the minimal amount that will provide enhanced properties, i.e. enhanced bonding to sintered valve metal powder with minimal adverse effect on electrical properties of finished anodes. An advantage of this invention is that during thermal sintering the concentration of the second metal, e.g. nickel, will initially be higher in the peripheral region to facilitate bonding of powders to the wire; and, as sintering continues the second metal will tend to diffuse away from the surface into the center of the wire to provide a low concentration at the peripheral region so as to not adversely affect electrical properties of an electrolytic capacitor.
In the method of this invention uniform coating of the second metal is preferably provided by using a solution comprising a soluble salt of the second metal. Such a solution can be coated onto the oxygen passivated surface of valve metal. The metal solution can be applied in an organic or aqueous solution. Useful anions for such soluble salts of such second metal include chloride, sulfate, carbonate and nitrate. Useful organic solvents include acetone, xylene, metanol, acetonitrile, methylene chloride, N-pyrrolidone, dimethylsulfoxide, dimethylacetamide, dimethylformamide, methyl ethyl ketone, glycol ethyl ether and the like. To assist in forming and maintaining a uniform coating on the surface the solution can preferably also include adjuvants such as binders, thickeners, leveling agents, surfactants and the like as are commonly employed in coatings. Useful adjuvant binders include polypropylene carbonate, hydroxypropyl methylcellulose, polyvinylalcohol, polyvinylbutyral and polymeric latex; a preferred adjuvant binder is polypropylene carbonate. The concentration of the second metal and adjuvant will depend on the thickness of a uniform coating that can be applied and the desired second metal concentration. Such solutions can be applied to the valve metal by any known method, e.g. dipping etc. A useful application for wire coating includes drawing wire through a porous medium saturated with the solution or through a bath of the solution. The coating is fixed by evaporating the solution and/or setting, e.g. gelling or crosslinking, a binder adjuvant. A useful way for fixing the coating on a wire is to draw the coated wire through a convection heating zone to assist in solvent removal. A useful convection heating zone can be a vertical or horizontal tubular chamber with a heated gas flow concurrent or countercurrent to the motion of the wire in the chamber.
In the method of this invention, after the second metal compound coating is fixed, the valve metal article is heated in the presence of an oxygen getter at a temperature sufficient to remove the oxide coating from the valve metal and to provide reduced second metal in the valve metal. Useful oxygen getters are materials having a greater affinity for oxygen than the valve metal. Such oxygen getters include magnesium, calcium, sodium, aluminum, carbon, titanium and zirconium. The getter must also have a higher affinity for oxygen than the second metal, e.g. when the second metal is titanium or zirconium. The oxygen getter can also be effective in extracting the anion from the second metal salt. Magnesium is a preferred oxygen getter when nickel chloride is used to provide the second metal salt. When the article is wire, the wire can be loosely wound on a spool, e.g. a valve metal spool, to provide exposure to the spooled wire to a gas phase comprising the getter material. The article is preferably exposed to the getter material in a vacuum furnace which allows evacuation of atmospheric oxygen and a relatively high concentration of the oxygen getter in an inert gas phase, e.g. gas phase comprising argon and magnesium vapor. When magnesium is used as the oxygen getter, temperatures above 800xc2x0 C. are useful for removing the surface oxygen from the valve metal. Procedures for removing oxygen from valve metals can be found in U.S. Pat. Nos. 3,647,420; 4,722,756; 4,960,471 and 5,241,481 all of which are incorporated herein by reference.
Valve metal wire prepared by this invention is typically softened by heat treatment with oxygen getter; the tensile strength of such softened wire is often unsatisfactory for use as lead wire in the manufacture of valve metal anodes for electrolytic capacitors. Heat treatment and quenching to harden the wire will generally be detrimental for use in capacitor applications due to oxygen pickup in such treatment. It has been discovered that the surface and peripheral region of second metal-containing valve metal wire of this invention is durable, unlike many surface coatings, and can tolerate wire drawing to a reduced diameter which effectively cold works the wire to provide sufficient strength for capacitor lead applications. In the case where the second metal salt coating was not uniformly applied, e.g. striated, and resulting in some peripheral region with a low concentration of the second metal, it may be useful to apply oxide coating to the wire prior to drawing to minimize galling from the drawing dies.
The above described process can also be modified to provide a second metal in valve metal powders, e.g. powders having dimensions on the order of 1 to 10 micrometers. For example, valve metal powder can be coated with second metal salt solution and dried e.g. in a fluid bed. Such metal salt-coated valve metal powder can be treated with an oxygen getter to form the second metal in the valve metal powder. Useful valve metal powders of this invention can include nickel-containing tantalum powder and niobium-containing tantalum powder and tantalum-containing niobium powder. A nickel-containing tantalum powder could be useful as a bonding agent for tantalum parts. Tantalum-containing niobium powder could provide enhanced electrical properties to niobium powders.
The following examples serve to illustrate certain aspects of this invention.